1. Field of the Invention
The present invention relates generally to downhole drilling operations. More particularly, the invention relates to tools for drilling boreholes. Still more particularly, the invention relates to reamer tools for enlarging boreholes during drilling operations.
2. Background of the Technology
An earth-boring drill bit is connected to the lower end of a drill string and is rotated by rotating the drill string from the surface, with a downhole motor, or by both. With weight-on-bit (WOB) applied, the rotating drill bit engages the formation and proceeds to form a borehole along a predetermined path toward a target zone.
In drilling operations, costs are generally proportional to the length of time it takes to drill the borehole to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times downhole tools must be changed or added to the drillstring in order to complete the borehole. This is the case because each time a tool is changed or added, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section-by-section. Once the drill string has been retrieved and the tool changed or added, the drillstring must be constructed section-by-section and lowered back into the borehole. This process, known as a “trip” of the drill string, requires considerable time, effort and expense. Since drilling costs are typically on the order of thousands of dollars per hour, it is desirable to reduce the number of times the drillstring must be tripped to complete the borehole.
During oil and gas drilling operations, achieving good borehole quality is also desirable. However, achieving good borehole quality when drilling long horizontal boreholes can be particularly challenging. In particular, to keep the borehole path as close as possible to horizontal, the driller may have to periodically change the direction of the borehole path because gravity has a tendency to cause the drill bit drop slightly below horizontal. Consequently, the driller must make corrections to lift the drill bit back up to horizontal with a directional motor or rotary steerable assembly. Unfortunately, these repeated corrections can result in the formation of ledges and/or sharp corners in the borehole that interfere with the passage of subsequent tools therethrough.
A reamer can be used to remove ledges and sharp corners in the borehole. For a non-expanding reamer, the diameter of the reamer is limited by the diameter of the casing in the borehole that the drill bit and reamer must pass through. If a concentric non-expanding reamer having the same or smaller diameter than the drill bit is used with the drill bit, the reamer will generally follow the path of the drill bit and may not be effective in removing the ledges and/or sharp corners. An eccentric reamer reams the borehole to a diameter that is larger than the diameter of the drill bit and is typically effective in removing ledges and sharp corners. Most conventional eccentric reamers have a plurality of straight circumferentially-spaced blades lined with cutter elements designed to engage and shear the borehole sidewall. The blades are non-uniformly distributed about the tool, and thus, occupy less than the total circumference of the tool, thereby making the reamer eccentric.
Conventional practice is not to use an eccentric reamer with a drill bit when drilling a new section of the borehole for fear of causing damage to the casing and/or cutter elements on the reamer blades. Consequently, after drilling a new section of the borehole, the driller will make a dedicated trip out of the borehole to couple an eccentric reamer to the drill bit and then trip back into the borehole with the drill bit and reamer in order to ream the previously created section of borehole. Alternately, the driller may complete drilling of the new section with the drill bit alone, trip out of the borehole, and then return into the borehole with the eccentric reamer to ream the hole. However, in both cases, an additional trip of the drillstring is required to ream the borehole.
During drilling operations, the drill bit may be rotated from the surface (e.g., with a top drive or rotary table) and/or rotated with a downhole mud motor. In drilling operations where the drill bit is rotated solely with the downhole mud motor (i.e., when sliding), an eccentric reamer is typically not used behind the mud motor. In particular, when sliding, the eccentric reamer does not rotate, and thus, cannot open the hole. Further, since an eccentric reamer is typically used with a drill bit having a diameter smaller than the inner diameter of the casing string (to allow the reamer to pass therethrough), a non-rotating eccentric reamer cannot pass through a borehole formed by such a drill bit.
Accordingly, there remains a need in the art for improved eccentric reamers for smoothing the profile of a borehole during drilling operations by removing ledges and sharp corners along the borehole sidewall. Such improved eccentric reamers would be particularly well-received if they were suitable for use in connection with a drill bit drilling a new section of borehole, as well for use in connection with drill bits rotated solely with downhole motors.